Semiconductor integrated circuits are becoming higher in density, higher in speed, and lower in power consumption, and what matters particularly in these devices is parasitic capacitance. For example, in a bipolar device, it is parasitic capacitance that is generated between collector and substrate, or in a MOS device, it is parasitic capacitance that is generated between source, drain and substrate. When this parasitic capacitance can be reduced, it is possible to compose a semiconductor device of higher speed and lower power consumption. Accordingly, as method of reducing this capacitance, many attempts have been made so far. Among others, there is an attempt of perfect isolation to replace parts immediately beneath and at sides of semiconductor device by insulators, and several prior arts relating to this attempt are described below.
In the U.S. Pat. No. 4,104,090, an anodically processed porous silicon is used to form an insulator layer immediately beneath the semiconductor region. In the first place, as the substrate, a P type silicon wafer having P.sup.+ -layer on the surface is used. Then P- or N-layer is formed on this P.sup.+ -layer by epitaxial growth. The surface of the silicon wafer is oxidized, and a proper opening is formed by photolithography. This opening is etched by reactive ion etching or other process to form a groove to reach said P.sup.+ -layer. To make this entire P.sup.+ -layer porous, the P.sup.+ -layer is selectively etched by the anodic process. The porous silicon is heated in oxygen and water ambient to become silicon dioxide completely. Finally the groove is filled with silicon dioxide or the like, and the P- or N-region separated by the insulator from the substrate and its surrounding is formed.
In the Japanese Laid-Open Patent No. 56-12749, a method of perfect insulation separation employing the conventional LOCOS technology is disclosed. First, a silicon nitride film is formed on a surface of silicon substrate, and a proper opening is formed by photolithography. In succession, using the patterned silicon nitride film as a mask, a sharp groove is formed in the silicon substrate. When the silicon nitride film is deposited on the silicon substrate surface and etched by sputtering, since the etching is excellent in linearity, only the silicon nitride film on the silicon substrate surface and groove bottom is etched, while the silicon nitride film is left on the side wall of the groove. Afterward, when thermally oxidizing for a proper time, the oxide film formed on the groove bottom reaches further to the lower part of the monocrystalline silicon region, and is finally joined with the oxide film propagating from the adjoining groove bottom. Thus, the monocrystalline silicon is completely isolated from the substrate.
The Japanese Laid-Open Patent No. 59-8346 refers to an improved version of perfect isolation technique of the preceding Japanese patent. More particularly, a band-like groove to reach the N.sup.+ - region is formed in the silicon substrate possessing an N.sup.+ buried region. Then, through this groove, the N.sup.+ -region is selectively etched, and the distance between adjacent grooves is properly determined. The N.sup.+ -region between the grooves is heated and oxidized until wholly turning into an oxide film, and it is completley isolated from the substrate.
As other method, for example, K. H. Nicholas et al. reported a process of using an orientation dependent etching in ELECTRONICS LETTER, Vol. 20, No. 24, 1985, pp. 1014-1015. In this method, first a silicon nitride film is deposited on a silicon (100) wafer, and a pattern is formed by photoetching. Next, using the nitride film as a mask, grooves are formed in the silicon substrate by reactive ion etching or other process. Furthermore, orientation-dependent etching is effected by ethylene diamine, and the distance between adjoining grooves is set to a proper size. Finally, the remaining narrow silicon-region between the adjoining grooves is heated and oxidized to be transformed into an oxide film so that the top silicon region is completely isolated from the substrate.